The present invention relates to a sensor for sensing any misalignment in position of a wafer on a wafer holder of an ion implantation machine following the wafer being fed onto the holder and before the wafer is clamped to the holder.
Present ion implantation machines have disc like structures with a plurality of wafer holder radially arranged about the center of the disc for holding a number of wafers, one of each holder. The holders each have a wafer pad upon which a wafer is loaded and a wafer clamp which is spring loaded and clamps down on the edges of the wafer to hold it firmly in place while being implanted. Two pins and two screws spaced around a periphery of the disc in conjuction with the clamp are used to hold a disc in position. Once the disc assembly is fully loaded it is moved by a transfer arm into a substantially vertical position in which it is rotated at speeds of rotation of up to 900 RPM while being implantated. Because of the heat generated on each wafer by the impinging ions which may have been accelerated through a voltage of up to 160 keV energy it is necessary to limit the time in the path of the accelerated ions as well as provide the wafers with a heat sink.
Often is happens that a disc will get misloaded and rest on one of the pins used to keep them in position. Once the loading clamps press down on the wafer the relatively fragile wafer breaks. If the broken wafer goes undetected once it is positioned facing the ion beam and spun at a high speed of rotation, the broken pieces fly around and break the other wafers mounted on the wafer assembly. In addition silicon dust particles become embedded in the ion implantation chamber and bombard subsequent loads of wafers during implantation leaving them with a pitted surface. Thus, the yields of implanted devices is considerably reduced.
Accordingly, it is a principal object of the present invention to provide an improved method of mounting wafer onto wafer holders. It is a further object to provide an apparatus for sensing any misalignment in the mounting of a wafer.